1. Field of the Invention
The present invention relates to a drive circuit for a stepping motor and, more particularly, to a drive circuit for a stepping motor which is capable of setting the exciting current to be supplied to each exciting coil at the optimum value.
2. Description of the Related Art
A stepping motor has hitherto been used in equipments such as a floppy disk drive (hereinunder referred to as "FDD") so as to obtain a necessary rotational angle. More specifically, in order to write or read necessary data by the access to a given track on a floppy disk, for example, it is necessary to position the head for reading and writing in such a manner as to be settled to the predetermined track on the disk. At this time, the position of the predetermined track on the disk can be represented by the rotational angle of the stepping motor with respect to the reference position thereof. In order to position the head at a given rotational position of the stepping motor, a stepping motor having a necessary stepping angle is used.
A stepping motor is composed of a rotor which rotates by the angle corresponding to the number of supplied pulses, and a plurality of exciting coils for generating excitation fields which act on the rotor. The stepping motor is driven by a drive circuit including a drive control circuit which supplies exciting currents to the exciting coils in correspondence with the pulse signals input from the outer host computer. To the drive circuit of the stepping motor, a pulse signal is input from the outside of the motor and the circuit supplies a predetermined exciting current to the exciting coils. The rotor is rotated by the excitation fields generated on the exciting coils in correspondence with the exciting currents. At this time, an exciting current is supplied to each exciting coil so that a predetermined rotational angle (hereinunder referred to as "stepping angle") is provided for the rotor every time one pulse signal is input to the drive circuit.
An example of the structure of such a drive circuit of a stepping motor is shown in FIG. 7. FIG. 7 shows the entire structure of a drive circuit for what is called four-phase two-coil type stepping motor. A drive circuit 10 is composed of a first exciting coil 14 and a second exciting coil for supplying magnetic excitation fields to a rotor 12, and a first drive control circuit 18 and a second drive control circuit 20 for supplying exciting currents I.sub.1 and I.sub.2 to the coils 14 and 16, respectively, in accordance with the pulse signals input from the outer host computer.
To the first and second drive control circuits 18 and 20, the exciting currents I.sub.1 and I.sub.2 are respectively supplied so that the rotor 12 rotates by a predetermined stepping angle every time one pulse signal is input from the outer host computer.
FIG. 8 is a circuit diagram of the first drive control circuit 18. Since the second drive control circuit 20 has a similar structure to that of the first drive control circuit 18, explanation of the structure of the second drive control circuit 20 will be omitted.
The first drive control circuit 18 is connected to both ends of the first exciting coil 14 and includes switching circuit 24 124 for outputting switching signal in accordance with the pulse signal input from, the outer host computer and transistors 126, 28 and 128 to which a driving voltage V.sub.cc is applied from the outer host computer and which are turned on/off in accordance with an ON/OFF signal supplied from the switching circuit 24 124.
In the drive control circuit 18, an ON signal is output from either of the switching circuits 24 and 124, and an OFF signal is output from the other switching circuit. When an ON signal is output from the switching circuit 24 and an OFF signal is output from the switching circuit 124, the transistors 26 and 28 are turned on and the transistors 126 and 128 are turned off. An exciting current I.sub.1a flows on the exciting coil 14 by the application of the driving voltage V.sub.cc. Reversely, when an OFF signal is output from the switching circuit 24 and an ON signal is output from the switching circuit 124, the reverse exciting current I.sub.1b flows on the exciting coil 14. In this way, according to the drive control circuit 18, it is possible to supply the exciting currents I.sub.1a and I.sub.1b to the exciting coil 14.
FIG. 9 shows the operation of a stepping motor using the drive circuit 10 having the structure shown in FIGS. 7 and 8.
FIG. 9(a) shows a stepping motor in which exciting currents I.sub.1 and I.sub.2 flow on the exciting coils 14 and 16, respectively, so that the rotor 12 rotates clockwise. If the stepping angle .theta..sub.s is set at 45.degree., the angular displacement .theta. from the reference radius upwardly extending from the center of the rotor 12 takes eight different values during one revolution of the rotor 12. FIG. 9(b) shows the relationship between the exciting currents I.sub.1, I.sub.2, the angular displacement .theta. and the stepping angle .theta..sub.s, and the number of pulse signals supplied from the outer host computer to the drive circuit 10.
When a pulse signal is input from the outer host computer in the state in which the angular displacement .theta. is 0, the exciting currents I.sub.1, I.sub.2 flowing on the exciting coils 14 and 16 take predetermined values of I.sub.1a and I.sub.2b, respectively. The predetermined value I.sub.1a is the value of an exciting current which is supplied to the exciting coil 14 when the transistors 26 and 28 are turned on in the first drive control circuit 18 shown in FIG. 8. The other predetermined value I.sub.2b is also the value of an exciting current which is similarly supplied to the exciting coil 16 in the second drive control circuit 20 having the same structure as the first drive control circuit 18. The rotor 12 is rotated clockwise by the stepping angle .theta..sub.s by the exciting fields generated on the exciting coils 14 and 16 in accordance with the exciting currents I.sub.1 and I.sub.2.
When another pulse signal is input from the outer host computer, the exciting currents I.sub.1 and I.sub.2 take the value of .theta. and the predetermined value I.sub.2b, respectively and the rotor 12 further rotates by the stepping angle .theta..sub.S.
When still another pulse signal is input from the outer host computer, the exciting currents I.sub.1 and I.sub.2 take the respective predetermined values I.sub.1b and I.sub.2b, and the rotor 12 further rotates by the stepping angle .theta..sub.S.
The predetermined value I.sub.1b is the value of an exciting current which is supplied to the exciting coil 14 when the transistors 126 and 128 are turned on in the first drive control circuit 18. The exciting current I.sub.1b is a current flowing in the reverse direction to the exciting current I.sub.1a.
According to the drive circuit 10, it is possible to obtain a stepping motor which is capable of outputting the rotational angle by the unit of the stepping angle .theta..sub.S, if necessary, by displacing the rotational angle of the rotor 12 by the unit of the stepping angle .theta..sub.S in accordance with the pulse signal which is input from the outer host computer in series.
However, the drive circuit for a conventional stepping motor having the above-described structure is disadvantageous in that nonuniformity in the resistances of the exciting coils or the like produces nonuniformity in the exciting currents supplied to the respective exciting coils, thereby producing an error in the stepping angle. For example, when the exciting currents have the following relationship due to nonuniformity, as shown in FIG. 9(b): EQU I.sub.1a &lt;I.sub.2b, I.sub.1b &lt;I.sub.2a
the nonuniformity in the exciting current produces a nonuniformity in the excitation fields and, hence, a nonuniformity in the attractions or repulsions. The stepping angle .theta..sub.S does not therefore agree with the desired stepping angle .theta..sub.S0 (45.degree. in FIG. 9(b)) and takes a value .theta..sub.S0 +.delta..sub.1 or .theta..sub.S0 -.delta..sub.2 which contains a stepping angle error .delta..sub.1 or .delta..sub.2 caused by the nonuniformity in the exciting currents. Such a drive circuit for a stepping motor causes a malfunction or defective operation of the equipment such as an error in writing or reading in equipments such as an FDD.